In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of water, an organic material such as a monohydric alcohol, a polyhydric alcohol or mixtures thereof.
The inks used in various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is molecularly dispersed or solvated by a carrier medium. A commonly used carrier medium is water or a mixture of water and organic co-solvents.
An inkjet recording element typically comprises a support having on at least one surface thereof an ink-receiving or image-forming layer, and includes those intended for reflection viewing, which have an opaque support, and those intended for viewing by transmitted light, which have a transparent support.
It is well known that in order to achieve and maintain photographic-quality images on such an image-recording element, an inkjet recording element must be readily wetted so there is no puddling, i.e., coalescence of adjacent ink dots, which leads to non-uniform density, exhibit no image bleeding, exhibit the ability to absorb high concentrations of ink and dry quickly to avoid elements blocking together when stacked against subsequent prints or other surfaces, exhibit no discontinuities or defects due to interactions between the support and/or layer(s), such as cracking, repellencies, comb lines and the like, not allow unabsorbed dyes to aggregate at the free surface causing dye crystallization, which results in bloom or bronzing effects in the imaged areas, and exhibit excellent image quality, and provide image fastness or stability to avoid fade from contact with water, ozone, radiation by daylight, tungsten light, or fluorescent light, or other environmental conditions that can otherwise cause image fade or deterioration.
Of particular relevance to the present invention, an inkjet recording element that simultaneously provides an almost instantaneous ink dry time and good image stability is desirable. However, given the wide range of ink compositions and ink volumes that a recording element needs to accommodate, these requirements of inkjet recording media are difficult to achieve simultaneously.
Inkjet recording elements are known that employ porous or non-porous single layer or multilayer coatings that act as suitable image receiving or recording layers on one or both sides of a porous or non-porous support. Recording elements that use non-porous coatings typically have good image stability but exhibit poor ink dry time. Recording elements that use porous coatings typically contain colloidal particulates and have poorer image stability but exhibit superior dry times.
There are generally two types of ink-receiving layers. The first type of ink-receiving layer comprises a non-porous coating of a polymer with a high capacity for swelling and absorbing ink by molecular diffusion. Cationic or anionic substances may be added to the coating to serve as a dye fixing agent or mordant for a cationic or anionic dye. This coating is optically transparent and very smooth, leading to a high glossy “photo-grade” receiver. The swellable binder forms a barrier to air-borne pollutants that otherwise may degrade the image dye over time. However, with this type of ink-receiving layer, the ink is usually absorbed slowly into the ink-receiving layer and the print is not instantaneously dry to the touch. Inkjet media having a non-porous layer are typically formed of one or more polymeric layers that swell and absorb applied ink. Due to limitations of the swelling mechanism, this type of media is relatively slow to absorb the ink, but once dry, printed images are often stable when subjected to light and ozone.
The second type of ink-receiving layer comprises a porous coating of inorganic, polymeric, or organic-inorganic composite particles, a polymeric binder, and optional additives such as dye-fixing agents or mordants. These particles can vary in chemical composition, size, shape, and intra-particle porosity. In this case, the printing liquid is absorbed into the open pores of the ink-receiving layer to obtain a print that is instantaneously dry to the touch. However, with this type of ink-receiving layer, image dyes adsorbed to the porous particles are relatively exposed to air and may fade unacceptably in a short time. In other words, the ink is absorbed very quickly into the porous layer by capillary action, but the open nature of the porous layer can contribute to instability of printed images, particularly when the images are exposed to environmental gases such as ozone.
In summary, the porous inkjet recording media have excellent drying properties, but generally suffer from dye fading, whereas, the swellable type of inkjet recording media may give less dye fading, but generally dry more slowly.
There remains a need for inkjet recording media having excellent drying properties and, at the same time, showing minimal dye fading. In addition, these inkjet recording media should preferably have properties such as good image density, as well as good image quality, preferably photographic image quality. It is towards fulfilling this need that the present invention is directed.
Mordant polymer particles containing cationic groups, for use in the image-receiving layer of inkjet recording elements, in order to mordant dye-based inks, are generally well known in the art. U.S. Pat. Nos. 6,045,917 and 6,645,582, for example, disclose water-insoluble cationic polymeric particles having at least about 20 mole percent of a cationic mordant moiety. Preferred mordants comprising a polymer having a vinylbenzyl trimethyl quaternary ammonium salt moiety are disclosed. U.S. Pat. No. 6,645,582 states that such particles can be core/shell particles wherein the core is organic or inorganic and the shell in either case is a cationic polymer.
Certain types of core-shell particles have been used in inkjet recording elements. However, the prior art does not disclose mordants in the form of core-shell particles that adequately address and solve the problem of dye fade.
U.S. Pat. No. 6,619,797 discloses an image-receiving layer comprising a cationic core/shell particle containing at least one ethylenically unsaturated monomer containing a trialkylammonium salt. However, the shell, but not the core, contains the trialkylammonium group.
U.S. Pat. No. 6,492,006 discloses an inkjet recording element comprising a support having thereon an image receiving layer comprising at least about 70% by weight of porous polymeric particles, the particles having a core/shell structure comprising a porous polymeric core covered with a shell of a water-soluble polymer. The recording element exhibited less cracking, but no improvement in dye density was disclosed. The porous polymeric particles do not have a monomer with cationic functionality, thus do not function as mordant.
US 2005/0031806 discloses a composition for forming an ink-accepting layer comprising a structured cationic core/shell latex, wherein a non-porous core does not have a cationic functional group and does not expand, and the shell contains a cationic functional group capable of expansion by an acid. The recording element exhibited improved absorption and water-fastness, but no improvement in dye fade was disclosed.
U.S. Pat. No. 6,818,685 discloses a coating composition comprising a non-ionic latex polymer (polyvinyl acetate), wherein the polyvinyl acetate has a core and a shell, and the shell comprises poly(vinyl alcohol). The particle core has no positive ionic character. A composition of high solids and low viscosity was disclosed and the recording element exhibited reduced dusting, but no improvement in dye fade as disclosed.
U.S. Pat. No. 6,969,445 and U.S. Pat. No. 6,669,815 describe graft copolymers of poly(vinyl alcohol) with cationic polymers.